The present invention relates, in general, to a storage system which is utilized in a computer system and the like, and more particularly to a storage system which is connected to host processors through cables each having a large connectable distance.
As for the technology relating to the architecture of transfer paths through which host processors and storage units provided in a computer system and the like are connected to one another, for example, there is well known the technology as described in an article "NIKKEI COMPUTER", published by NIKKEI BP Co. Ltd., Sep. 24, 1990, pp. 57 to 58. This article discloses the technology relating to a storage system in which host processors and storage units are connected to one another through optical fiber cables. According to this article, it is shown that the host processors and the storage units are connected to one another through the optical fiber cables in such a way, whereby the connection distance between the associated host processor and storage unit can be extended up to 9 km at a maximum. That is, the optical fiber cable is employed for the connection between the host processor and the storage unit, whereby the connectable distance thereof can be increased remarkably up to the order of km.
On the other hand, in HITACHI MANUAL: HITAC H-6581-C3 TYPE DISC CONTROLLER, H-6587 TYPE DISC DRIVER, H-6586 TYPE DISC DRIVER, H-6598 TYPE DISC DRIVER, H-8598 TYPE DISC DRIVER, 8080-2-130-10, pp. 138, there is described the data transfer method for use in a disc unit which is a typical one of the storage units employed in the computer system.
Normally, with the disc unit, prior to the data transfer, the positioning processing, such as the seek processing and the rotation wait processing, of positioning a head for reading/writing the data in a data recording position on a disc as the recording medium needs to be executed. While executing the positioning processing, both the data transfer paths distributed between the controller and the host processors, and the data transfer paths distributed between the controller and the disc units are released. Thereafter, when the head of the disc unit approaches to the recording position in which the data is intended to be read/written, those data transfer paths which have been released are occupied again (hereinafter, this processing is referred to as "the reconnection", when applicable), and then the processing of reading/writing the data begins to be executed.
In the above-mentioned prior art, by employing the optical fiber cables, the connection distances between the host processors and the storage units are increased. However, by increasing the connection distances between the host processors and the storage units, the time required for the data to be transmitted through the cable, i.e., the delay time, which has not become the great problem conventionally can not be disregarded as compared with the time required for the data to be transmitted to/from the storage unit. However, in the above-mentioned prior art, such delay time was not taken into consideration at all. For example, under the conditions in which the data transfer between the host processors and the associated disc units was performed on the basis of the electric signals, and the lengths of the cables, through which the host processors and the disc units were connected to each other, are uniformly short, the timing of starting the execution of the reconnection which was carried out after the completion of the positioning processing for the disc unit corresponded to the time point when the head of the disc unit was always positioned before the recording position of the data to be read/written by a fixed distance. However, if the length of the cable through which the host processor and the disc unit are connected to each other is increased, there is encountered the situation in which the time required for the host processors to inform of start of the execution of the reconnection is necessarily increased, and on completion of the reconnection, the head of the disc unit passes the position where the data, which is an object of read/write, is to be recorded. As a result, there arises the problem that it is necessary to wait the head to be positioned in the position, where the data is to be recorded, again, and thus the performance is degraded remarkably.
In addition, there is an example of the inconvenience which occurs when the input/output requests (the I/O requests) are issued competitively from a plurality of host processors to the same storage unit. For example, let us consider that in the state in which while the storage unit executes the processings for the I/O request issued from a certain host processor, the storage unit receives other I/O requests issued from other two or more host processors and those requests are waited to be processed, the processings for the I/O request in execution has been completed. In such a case, normally, the storage unit of interest posts the fact that the next I/O request becomes executable to the host processors, and then processes the I/O request which has been received first from the host processor. In this case, under the conditions in which the cables through which the host processors and the storage units are connected to one another are uniformly short in length, if the I/O request which has been received first from the host processor is next executed, there arises in particular no problem. However, if the range of the length of each cable is increased, and thus the cables distributed between the host processors and the storage units become different in length from one another, the difference occurs in the time required for the notification, sent from the storage unit, for posting that the next I/O request is executable to reach the host processors. Therefore, there is the possibility that in the above-mentioned method, the I/O request issued from the host processor, which has the larger connection distance to the associated storage unit, is always waited to be processed, and if the worst comes to the worst, that host processor falls into the situation in which it can not receive the service.